The presence of liquid contaminants in flowing gas streams poses a serious problem if the contaminating substance renders the gas unfit for use or unfit for storage. If the contaminant is toxic or polluting, such as polychlorinated biphenyls, it is necessary to remove the contaminant almost entirely (less than 50 parts per million or ppm in gas or any associated liquid) to yield a usable and non-polluting gas product.
Of particular interest are natural gas transmission lines contaminated with polychlorinated biphenyls (PCBs). The presence of PCBs in natural gas transmission lines is especially troublesome since PCBs, in addition to being among the most degradation resistant compounds known, have shown chronic effects, including carcinogenicity, in many living species. These chronic effects have been observed at very low PCB exposure levels, and therefore PCBs have been declared a general threat to the environment by appropriate government agencies. Although PCB production is now banned, PCBs, due to their inherent superb dielectric properties, remain in use as fluid insulators for power transformers. The transformers are totally enclosed, therefore the PCBs are isolated from contact with the environment. Of an estimated total production of 2.5 billion pounds, about 1.5 billion pounds remains in electric power service. However, in addition to a high dielectric constant, PCBs also exhibit a thermal stability and a degree of lubricity making them useful as additives in extreme pressure lubricants, such as compressor oils. As a result, before the inherent toxicity of PCBs was known and before the subsequent PCB production ban, a significant amount of polychlorinated biphenyl was used in compressor lubricating oil blends. Residual lubricating oils of this type now provide a source of PCB contamination in natural gas pipelines.
Ultimately, operators of natural gas transmission lines have had utility companies and industrial users refuse delivery of PCB-contaminated natural gas, fearing perpetual PCB contamination of underground storage reservoirs and facilities. This refusal to store excess contamination-suspected natural gas supplies adversely affects pipeline operations and load factors. The customers will use contaminated gas only as needed; therefore, by not drawing from storage supplies, peak usage periods strain the pipeline's ability to supply the required gas, while lull periods may essentially shut down the pipeline. Removal of PCBs from the natural gas would allow pipeline users to store the decontaminated gas in underground reservoirs, thus providing enough available gas for peak usage periods, and allowing the storage operators to replace this reserve during low usage periods. The load factor on the pipeline would be improved, and its operation would be more efficient.
Routine attempts are made to remove condensed liquids from gas pipelines. However, complete separation is rarely accomplished, especially when flow rates are high and pressures are elevated, as in cross-country natural gas transmission lines having variable flows and pressures on the order of 250 to 1000 psig. Presently, liquids are removed from natural gas transmission lines by draining liquid traps or drips, manually or automatically. This method is acceptable for removal of condensed liquids unless contaminants such as PCBs are present, whereby some aerosol-sized mists of extremely low mass continue to flow with the gas stream.
Total removal of liquids, including aerosol-sized liquid contaminants from gas streams, can be accomplished by adsorption of the liquid contaminants onto the surface of a highly porous solid. After saturation with liquid contaminant, the adsorbent can be replaced or heat-regenerated. The inherent disadvantage of this method is the entire system must be temporarily closed to effect adsorbent replacement, or duplicate facilities are required, allowing one adsorption unit to be in operation while the other is being cleaned and regenerated. A further disadvantage is the expense of regenerating or disposing of a large volume of spent absorbent, especially if it is saturated with a highly toxic contaminant such as PCBs.
An alternate method involves chemical dechlorination of the PCB-contaminated liquid by strong reagents such as sodium oxalate, sodium glycolate or sodium naphthalene. These strong reagents strip the chlorine atoms from the polychlorinated biphenyl molecule, converting the PCB into non-toxic biphenyl. For use in natural gas transmission lines, the dechlorinating agent must be dissolved in a polar liquid, such as water or methanol, then applied to an extended surface of a porous material. This method has a disadvantage in that the porous surface must be kept moist to achieve reaction between the PCBs in the gas flow and the dechlorinating agent. In addition, a gas treated by aqueous reagents requires dehydration before its return to the pipeline. The net result of this method is generation of relatively non-toxic biphenyl--a thick liquid of pungent odor adsorbed on the extended surface. The spent adsorbent then may be discarded properly or regenerated. Although the disadvantage of duplicate facilities remains with this system, disposal of relatively non-toxic biphenyl is considerably easier and more economical than disposal of the toxic PCBs. Nevertheless, the operational use of this PCB removal method poses significant economic and logistical problems, generally due to the volume of solvent required to keep the porous surface moist during the reaction.
The process of the present invention overcomes the above-mentioned problems. In accordance with the present invention, low vapor pressure, film-forming contaminants are removed in a continuous process, from flowing gas streams to an unexpected degree. The method of this invention is especially effective in removing contaminants, such as polychlorinated biphenyls and higher molecular weight hydrocarbons containing at least ten carbon atoms, from flowing gas streams. The contaminants are essentially completely removed in a three-stage process, constructed to reduce the contaminants to successively smaller concentrations at each stage. Then, contrary to present practice and to assure complete contaminant retention and removal, the separated contaminant is collected within each stage by a relatively large amount of an intentionally added solvent that is miscible with the contaminant.